These days precision metal machining such as drilling, tapping, boring, milling, cutting of inner or outer diameters or surface grinding usually relies on CNC (Computer Numerical Control) machine tools in cooperation with a plurality of cutters or tools. Those cutters or tools mostly have an elongated stem connecting to a driving spindle of the CNC machine tools through a chucking apparatus or chucking structure. The driving spindle rotates in high speed to drive the cutters or tools held by the chucking apparatus to perform machining on a targeted object.
Hence the holding condition of the chucking apparatus on the cutters or tools directly affects the machining precision of the targeted object. If clamping is not tight enough during high speed rotation of the machining tool idle rotation of the cutters or tools occurs. Referring to FIG. 1, a conventional chucking apparatus mainly includes a fastening member 1, a collet 2 and a holding member 3. The holding member 3 is connected to a driving spindle (not shown in the drawing) of a machining tool, and has a housing space 4 to hold the collet 2 to form a tight coupling. The collet 2 is a hollow conical element and has a plurality of conical pawl structures 5 formed in an annular manner with a center passage 6. The pawl structures 5 are elastic. When a cutter or tool is inserted into the collet 2 the inner diameter of the center passage 6 can be adjusted accordingly. The fastening member 1 and the holding member 3 have respectively a corresponding screw thread 7 and 7′ for screwing together to firmly hold the collet 2. In addition, as the collet 2 has the pawl structures 5, when the holding member 3 and the fastening member 1 are screwed together, the pawl structures 5 are forced and shrunk inwards to strengthen coupling of the cutter or tool to increase the firmness of machining and enhance the machining precision of the targeted object.
The collet 2 equipped with the pawl structures 5 to hold the cutter or tool has many advantages, such as it can hold cutters or tools of varying sizes and specifications. Moreover, when the holding member 3 and the fastening member 1 are screwed together, they automatically clamp the cutter or tool in an optimal condition. However, when the conical top of the collet 2 is moved towards the holding member 3 it mainly holds the distal end of the cutter or tool. Due to the targeted object to receive machining usually is a hard article such as metal, and the driving spindle (not shown in the drawing) of the machining tool rotates at high speeds, with the collet 2 holding the cutter or tool at the distal end, only the distal end receives the force, hence the cutter or tool is not being held and fastened securely. Loosening or slipping of the cutter could take place during machining process. Moreover, as the cutter or tool has an excessive portion exposed outside the collet 2, when a great force is applied or the forcing angle is not applied vertically, the cutter or tool could be skewed. As a result, machining precision suffers. It could even cause damage of the collet 2 or the machining tool and result in a great loss of cost.
U.S. Pat. No. 1,953,637 discloses a chuck which has a shell, a nut coupling with the shell and a stem running through the nut and movable relative to the nut. The stem has a plurality of springs to couple with a plurality of jaws. The jaws are held in the shell. The stem moves towards the shell to push the jaws through the springs so that the jaws butt the inner wall of the shell and the front end thereof can grip a cutter. However, when the jaws butt the inner wall of the shell, the rear end of the jaws (namely one end connecting to the spring) is merely braced by the elasticity of the spring. The rear end of the jaws cannot firmly grip the cutter, and the cutter is not firmly held. When the cutter performs machining to a work piece, cutter wobbling could occur due to infirm holding. This could cause poor precision of machining and produce rough edges on the work piece. The cutter could even be thrown out during high speed operation. Not only the cutter or work piece could be damaged, it also creates a serious concern on user's safety.